The present invention relates to flashing fluids extracted from pressurized reactor vessels and particularly to flash tanks for flashing black liquor from a pressurized reactor vessel in a pulping or biomass treatment system.
Flash tanks are generally used to flash a high pressure fluid liquor stream including steam and condensate. A flash tank typically has a high pressure inlet port, an interior chamber, an upper steam or gas discharge port and a lower condensate or liquid discharge port. Flash tanks safely and efficiently reduce pressure in a pressurized fluid stream, allow recovery of heat energy from the stream, and collect chemicals from the stream in condensate.
Flash tanks may be used to recover chemicals from chemical pulping systems, such as Kraft cooking systems. Flash tanks are also used in other types of cooking systems for chemical and mechanical-chemical pulping systems. To pulp wood chips or other comminuted cellulosic fibrous organic material (collectively referred to herein as “cellulosic material”), the cellulosic material is mixed with liquors, e.g., water and cooking chemicals, and pumped in a pressurized treatment vessel. Sodium hydroxide, sodium sulfite and other alkali chemicals are used to “cook” the cellulosic material such as in a Kraft cooking process. These chemicals degrade lignins and other hemicellulose compounds in the cellulosic material. The Kraft cooking process is typically performed at temperatures in a range of 100 degrees Celsius (100° C.) to 170° C. and at pressures at or substantially greater than atmospheric.
The cooking (reactor) vessels may be batch or continuous flow vessels. The cooking vessels are generally vertically oriented and may be sufficiently large to process 1,000 tons or more of cellulosic material per day. The material continuously enters and leaves the vessel, and remains in the vessel for several hours. In addition to the cooking vessel, a conventional pulping system may include other reactor vessels (such as vessels operating at or near atmospheric pressure or pressurized above atmospheric pressure) such as for impregnating the cellulosic material with liquors prior to the cooking vessel. In view of the large amount cellulosic material in the impregnation and cooking vessels, a large volume of black liquor is typically extracted from these vessels.
The black liquor includes the cooking chemicals and organic chemicals or compounds, e.g., hydrolysate, residual alkali, lignin, hemicellulose and other dissolved organic substances, dissolved from the cellulosic materials. The black liquor is flashed in a flash tank to generate steam and condensate. The cooking chemicals and organic compounds are included with the liquid condensate formed when the liquor is flashed. The steam formed from flashing is generally free of the chemicals and organic compounds. The condensate is processed to, for example, recover and recausticize the cooking chemical. The steam may be used as heat energy in the pulping system.
In conventional flash tanks, the black liquor enters flash tanks through an inlet pipe having a fixed inlet diameter. The inlet is not variable or otherwise controllable to adjust the size of the black liquor flow passage. Changes to the flow passage at the inlet to a conventional flash tank for black liquor have been made by changing the inlet piping to the flash tank. Conventional flash tanks do not have a means for adjusting the flow passage; controlling of the volume or the velocity of the black liquor flow into the flash tank, pressure drop in the flash tank, or regulating the pressure in the conduits containing black liquor connected to the inlets to the flash tanks.